build: only skip hash rewriting for sandboxing on linux
build: only skip hash rewriting for sandboxing on linux

The sandbox on darwin, and possibly other future platforms, doesn't have
enough isolation to redirect outputs without hash rewriting.

pikajude/nix,pikajude/nix,vcunat/nix,rycee/nix,NixOS/nix,NixOS/nix,vcunat/nix,ehmry/nix,NixOS/nix,ehmry/nix,dtzWill/nix,dtzWill/nix,ehmry/nix,rycee/nix,dtzWill/nix,vcunat/nix,NixOS/nix,ehmry/nix,dtzWill/nix,vcunat/nix,ehmry/nix,ehmry/nix,rycee/nix,dtzWill/nix,pikajude/nix,rycee/nix,NixOS/nix,rycee/nix